The present invention relates to an end plate provided at an end in the cell stacking direction of a cell stack body of a fuel cell stack. The end plate has circulation holes for circulating fluid to and from passages of the cell stack body.
A fuel cell has a fuel cell stack (for example, refer to Japanese Laid-Open Patent Publications No. 2016-134335 and No. 2016-96032). The fuel cell stack includes a cell stack body constituted by plate-shaped single cells stacked in the thickness direction, a pair of end plates holding the cell stack body in between in the cell stacking direction, and connecting pipes connected to the end plates.
Conventional end plates, such as those disclosed in Japanese Laid-Open Patent Publications No. 2016-134335 and No. 2016-96032, include circulation holes for circulating anode gas, cathode gas, and cooling medium (hereinafter, collectively referred to as fluid) to and from the passages formed in the cell stack body. The end plate includes a metal plate body having through-holes constituting the circulation holes and inner circumference covering portions, which cover the inner circumferential surfaces of the through-holes. The plate body is made of, for example, aluminum. The inner circumference covering portions are made of an electrically insulating plastic such as polypropylene. The inner circumference covering portions are formed integrally with the plate body by injecting molten plastic into a mold with the plate body inserted into the mold.
Japanese Laid-Open Patent Publication No. 2016-134335 discloses that it is preferable to set the cross-sectional shape of the passages of the cell stack body to a rectangle to increase the cross-sectional flow area, thereby reducing the pressure loss of the fluid. The publication also discloses that it is preferable to set the cross-sectional shape of the connecting pipes connected to the end plate to a circle to reduce the pressure loss of the fluid flowing through the connecting pipes. Refer to paragraph [0025] of the publication. Furthermore, the publication discloses that it is preferable to set, to a rectangle, the cross-sectional shape of the section of each inner circumference covering portion that is opposed to the passage of the cell stack body, set, to a substantially perfect circle, the cross-sectional shape of the section of each inner circumference covering portion that is opposed to the connecting pipe, and gradually change the cross-sectional shape of the intermediate section of each inner circumference covering portion from a rectangle to a substantially perfect circle.
The plate body of the end plate disclosed in Japanese Laid-Open Patent Publication No. 2016-96032 includes a main portion having through-holes and a flange portion protruding away from the outer circumference of the main portion. The flange portion has a fastening surface, to which the flange of the case accommodating the cell stack body is fastened (see FIGS. 7, 8 and 10 of the publication). Further, in the end plate of the publication, the outer circumferential surface of the main portion is covered by an outer circumference covering portion, which is integrally formed with the inner circumference covering portions, while the outer circumferential surface of the flange portion is not covered with plastic and is exposed.
In the end plate disclosed in Japanese Laid-Open Patent Publication No. 2016-96032, the cross-sectional shape of each inner circumference covering portion may be gradually changed from a rectangle to a substantially perfect circle as in the end plate disclosed in Japanese Laid-Open Patent Publication No. 2016-134335. This configuration would, however, disadvantageously complicates the shape of the through-holes and the mold and limit the cross-sectional flow area since the section of each inner circumference covering portion that is opposed to the connecting pipe has the shape of a substantially perfect circle, resulting in an increased pressure loss.
On the other hand, if the cross-sectional shape of the section of each inner circumference covering portion that is opposed to the connecting pipe is made rectangular, the shape of the circumferential edge of the corresponding through-hole at the end facing the connecting pipe (hereinafter, referred to as an outer end) will be rectangular. In this case, due to the injection molding pressure of plastic at the time of insert molding, the stress tends to concentrate at the corner portions of the circumferential edge of the outer end of the through-hole, which can deform the circumferential edge of the outer end of the through-hole. Thus, in order to prevent deformation of the plate body, additional measures such as an increased thickness of the plate body are required.